handplane mechanics – one planemaker’s breakdown

Several years back, Thomas Lie-Nielsen was quoted as saying “A plane is just a jig for a chisel”. It’s stuck with me a long time, because it’s so right – but also because I think it can be somewhat misleading, depending on how you look at it. The confusion point is the word ‘just’. Just implies simplicity and a certain dispensability that I don’t think is particularly illuminating… because while a plane really is a jig, it’s a jig of spectacularly coherent development, and the result of centuries of thought and design by some truly brilliant people. That ‘just’ obscures an awful lot of specifics that go into that jig – and no one knows this more than Lie-Nielsen – and so I’d like to delve a little bit into how I’ve come to think about planes, and the factors that I think have to be considered to really make one well. I should also say that I’m primarily addressing bench planes here, but the principles still hold in some form or another for joinery and specialty planes as well…

As I think of it, there are 3 basic factors at work in a plane: the sharpness of the edge doing the cutting, the rigidity of the edge transport, and the operation of the plane body on the fibers of the wood as the blade does its job.

First – Sharpness is the simplest to address, but clearly the most important – and in my experience is probably the most common problem area users have. Basically, if you cannot sharpen well, and consistently, even the best plane won’t work well. And a sharp edge can make up for a lot of shortcomings in the other areas. I’d really like to talk for hours about sharpening here, but I’m not going to other than to say that it takes practice, attention, and work. Find a method that someone who knows what they’re doing uses, and practice it. For a year. Then you’ll know what sharp is.

You back already?

OK – then lets get on to the next factor: edge transport. In most planes this is essentially about the relationship between the blade and wood, and the consistency of that relationship. The relationship is most simply understood as the effective pitch of the blade, and is determined by the angle between the sole and the first few thousandths of an inch of the cutting edge. That’s it.

The consistency of the blade-wood relationship, however, is much much more interesting – and is quite often the factor that separates a decent or poor plane from a really great one. The easiest way to think of the ‘relationship’ I’m talking about is to consider just how doggedly the blade edge keeps its position with respect to the wood. There are a lot of factors that go into this, including the flatness and slickness/stickiness of the plane sole; the mating of the body (sole) to the blade bed (frog); the mating of the bed and blade; the mechanism (lever cap, wedge, screw) that secures the blade to bed; and finally the blade itself, where the type of steel (or laminations), honing angle, and blade thickness all affect how consistently the edge can hold its position.

Finally, we come to the part of planes that I think is least understood or explicitly considered: fiber manipulations. The well-designed plane also has features designed to control the action of the wood fibers both before and after the cut. The flatness of the plane sole counts here as well, as it adds compression to the wood prior to the cut. The next factor is the mouth – a really tight mouth means that the sole’s compression is continuous up to within a few thousandths of an inch of the cut. This matters because if tearout is going to happen, it will be largely limited by the compression. This is why a tight mouth can have such an effect on a plane’s performance. The sharpness of the edge also affects the fibers – a sharp edge will tend to sever the fibers sooner, and therefore it has less tendency to ‘lever’ the fibers out than a dull blade. Leverage effects lead to tearout, so edge sharpness also affects this aspect of performance.

Pressing on, there are also effects available to a plane designer after the cutting edge, starting with pitch. Blade pitch – or the effective angle of the cutting edge – will have a significant impact on whether the fibers of the wood tend to fold and break, or whether they remain intact. Intact is bad, because if the fibers stay rigid after the cutting edge, the tendency is to increase leverage on the fiber – leading, again, to tearout ahead of the blade. Higher pitch angles tend to compress and fold the fibers more readily than lower pitches, which is why york (50 degrees), middle pitch (55 degrees) and half pitch (60 degree) planes are less likely to tear out than 45-degree standard pitch planes. Please note that for these effects, the plane really doesn’t care if the pitch is due to the bed angle (in a bevel-down plane) or is the result of bed angle PLUS blade bevel angle (as in a bee-down plane). There are differences between these applications, but they’re for a different discussion.

This brings us to the use of a secondary iron, otherwise known as a cop iron or chip breaker. This approach gained a lot of steam recently after the ‘rediscovery’ of research done on supersurfacers in japan, but it’s a strategy for preventing tearout that’s been in use constantly for well over a century, both in the west and in Japan. Essentially, without raising blade pitch, a secondary edge just behind the cutting edge is employed to ‘break’ severed fibers, preventing them from levering prior to the cutting edge. This is performing the same function as increased pitch, but uses a different set of criteria. It has the advantage that you retain the ease of pushing the plane and the relatively cleaner surface of a lower cutting angle, but with the tearout reduction of a much higher pitch. The downside to cap irons is the more complicated geometry and setup, since the breaker must be within 4-8 thousandths of an inch of the cutting edge to be effective. With practice this should prevent no significant hurdle to most woodworkers, however, and this strategy is particularly economically useful as renders standard pitch Stanley and other commonly available vintage planes incredibly effective.

So this begs the question of what is the best strategy for designing planes. The answer is ‘Yes.’ And: ‘it depends.’

Here’s the thing – it might be tempting to say there is one ideal solution to designing planes for minimal tearout – but if that were the case, then plane design would be simple and boring and I wouldn’t be doing it.

There are tradeoffs to every possible design. Higher effective blade angles (pitch) gives better tearout reduction, but can also leave a rougher surface than lower angles. Using a chip breaker can reduce tearout impressively, but it somewhat interferes with using an extremely tight fixed mouth – and it can be difficult to master for the uninitiated. Using very tight fixed mouth openings works well, but it creates a plane that is really only good for final smoothing – there’s no hogging material off at 3 thou shaving limits.

Personally, I make planes that are really only designed for final smoothing and some light joinery – so using tight mouths with single irons makes sense for me. I also have the luxury of having many planes, so very effective one-trick ponies like infill smoothers are quite reasonable. For someone learning, or who prefers to keep a very light toolkit, cap irons or higher pitches are going to be a better solution.

So there. Now I’ve made the simple complex again – lucky you, dear reader, lucky you.